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Abstract

The carrying capacity for cod in a Norwegian fjord was analysed by means of a simulation
model. Four age groups of cod were represented as well as the maln prey groups labrids, gobies and
benthic organisms. These groups made up a near-shore compartment of the model, while nutrients,
phytoplankton, herbivorous, carnivorous and gelatinous zooplankton comprised a pelagic compartment.
The 2 compartments were coupled through water exchange. The pnmary production was driven
by solar radiation, temperature, freshwater runoff, and deep water convection. Water exchange across
the sill regulated the magnitude of advection of secondary producers serving as prey for higher trophic
levels. Generally, simulation models including predation are sensitive to the feeding representations,
and we propose a new method for obtaining estimates of the 'half-saturation' parameter. Simulations of
1 yr indicate that cod production is sensitive to the amount of zooplankton advected into the fjord and
to the availability of benthic preys. Under good environmental conditions maximal cod production is
estimated to be 0.5 to 1.3 X 10' ind. yr-', which is 7 to 8 times higher than the average levels of natural
cod recruitment. Recruitment beyond 0.5 to 1.3 X 106 ind. yr-' will reduce the overall cod production
because of food shortage and cannibalism. Validity of the model is indicated, as good agreement
between predicted and observed diet composition of the cod is demonstrated. The 2 main implications
from the present simulations regarding extensive cod mariculture are: (1) optimal cod production is
obtained if the sum of released and wild recruits is within the range of the carrying capacity of
juveniles; and (2) that releases of the same number of juveniles in several years gives dissimilar cod
production due to interannual variations in the magnitude of advection, and its influence on zooplankton
availability in Masfjorden.